A fuel cell is a next-generation power generation system which is expected to be installed and widely used from the viewpoints of energy conservation and concern for the environment, since a fuel cell utilizes energy generated during a reaction combining hydrogen and oxygen. There are several types of fuel cells including a solid electrolyte type, a molten carbonate type, a phosphoric acid type, and a solid polymer type.
A solid polymer fuel cell has gathered particular attention for use as a power source for an electric vehicle and a dispersed power source for household use, since a solid polymer fuel cell can be easily downsized because of its high power density, and it is easy to start and stop because of its relatively low operating temperature compared to other types of fuel cell.
FIG. 1 shows a structure of a solid polymer fuel cell, which may be referred to as a fuel cell. FIG. 1(a) is an exploded view of a unit cell of the fuel cell, and FIG. 1 (b) is a perspective view of a fuel cell formed by assembling a number of the unit cells.
As shown in FIG. 1, a fuel cell 1 consists of a stack of unit cells. Each unit cell comprises, as shown in FIG. 1(a), a solid polymer electrolyte membrane 2, a gaseous diffusion electrode layer 3 functioning as a negative electrode, which may be referred to below as a fuel electrode or an anode, the anode being laminated on one surface of the solid polymer electrolyte membrane, a gaseous diffusion electrode layer 4 functioning as a positive electrode, which may be referred to below as an oxidant electrode or a cathode, the cathode being laminated on the other surface of the solid polymer electrolyte membrane, and separators (bipolar plates) 5a and 5b which are laminated on both outer surfaces of the gas diffusion electrode layers.
A fuel cell may be a water-cooled type comprising a water separator having a passageway for cooling water which is disposed between the unit cells or disposed at an assembly of several unit cells. The present invention relates to such a water-cooled fuel cell.
The solid polymer electrolyte membrane 2, which may be referred to below as an electrolyte membrane, is formed of a fluorine-type proton-conducting membrane having a proton-exchange group. The anode 3 and the cathode 4 may comprise a catalyst layer containing a particulate platinum catalyst and graphite powder, and optionally a fluorine resin having a proton-exchange group. In this case, the reaction for generating power is promoted by contacting this catalyst layer with a fuel gas or oxidizing gas.
Fuel gas (hydrogen or hydrogen-containing gas) A is distributed through passages 6a provided in the separator 5a to supply hydrogen to the fuel electrode membrane 3. Oxidizing gas B such as air is distributed through passages 6b provided in the separator 5b to supply oxygen. Direct current power is generated by an electro-chemical reaction caused by supplying these gases.
A separator for a solid polymer fuel cell needs to perform the following functions.
(1) A function as a path uniformly distributing fuel gas or oxidizing gas in the surface of a cell,
(2) a function as a path efficiently exhausting water formed in the cathode side with carrier gases such as air and oxygen from a fuel cell,
(3) a function as an electrical path by contacting electrode membranes (anode 3, cathode 4), and further as an electrical connector between unit cells,
(4) a function as a partition wall between an anodic chamber of one unit cell and a cathodic chamber of an adjoining unit cell, and
(5) a function as a partition wall between a water-cooling passageway of a water-cooled fuel cell and a unit cell adjacent to the water-cooling passageway.
The substrate of such a separator for a solid polymer fuel cell, which will be referred to below as a separator, is roughly classified as a metallic material or a carbonaceous material.
A separator of a metallic material such as stainless steel, titanium, and carbon steel is produced by a process such as pressing. On the other hand, a separator of a carbonaceous material is produced by several different processes. Examples of such processes include a method of firing a graphite substrate in which a thermosetting resin such as a phenol resin and a furane resin is impregnated, and a method of forming a glassy carbon by mixing a carbon powder with a phenol resin, a furane resin, tar pitch, or the like, press molding or injection molding the resulting mixture to form a planar member, and sintering the resulting molded member.
A metallic material such as stainless steel has the advantage that the weight of a separator can be reduced since this material has a high degree of machinability derived from its being a metal, and hence the thickness of a separator can be reduced. However, the electroconductivity may be reduced by elution of metal ions due to corrosion or oxidation of the surface of the metal. Therefore, a separator formed of a metallic material, which is referred to below as a metallic separator, has the problem that contact resistance between a metallic separator and a gaseous diffusion electrode layer may increase.
On the other hand, a carbonaceous material has the advantage that the weight of the obtained separator is small. However, a separator formed of a carbonaceous material has problems such as high gas-permeability and low mechanical strength.
As one method for solving the above-described problem of a metallic separator, it is proposed in Patent Document 1 that the contact surface of a metallic separator with an electrode be coated with gold plating. However, utilizing a large amount of gold for vehicles such as cars and fixed fuel cells is problematic from the viewpoints of economic efficiency and availability of resources.
Therefore, it has been proposed to coat the surface of a metallic separator with carbon to resolve the above-described problem without using gold.
The following technologies relating to a method of coating the surface of a metallic separator with carbon have been proposed.
(A) The material of a painted metallic separator for a solid polymer fuel cell disclosed in Patent Document 2 comprises a substrate formed of an austenitic stainless steel surface which has been acid-washed and an electroconductive paint film having a thickness of 3 to 20 micrometers on the substrate. An electroconductive agent in the paint film is a mixture of graphite powder and carbon black. This patent document discloses a process in which the surface of a substrate of a metallic separator is washed by an acid and the surface of the substrate after acid-washing is coated with an electroconductive paint containing carbon.
(B) Patent Document 3 discloses a paint for a separator for a fuel cell which contains graphite as an electroconductive material and which is capable of forming an electroconductive paint film by coating he surface of a metallic or carbonaceous separator for a fuel cell with the paint. This paint contains a binder consisting of a copolymerized material (VDF-HFP copolymer) of vinylidene fluoride (VDF) and hexafluoropropylene (HFP) in an amount of 10 percent by weight or more, and a solvent compatible with the binder. The ratio by weight of the content of the electroconductive material and the content of the binder is 15:85 to 90:10, and the content of the solvent is 50 to 90 percent by weight.
(C) Patent Document 4 discloses a separator for a fuel cell forming a gas passageway together with a planar electrode of a unit cell. The separator comprises a metallic plate having low electric resistance and an amorphous carbon film which covers the metallic plate and forms the surface of the gas passageway. The hydrogen content of the amorphous carbon film CH is 1 to 20 atomic percent. This document proposes a method of forming a carbonaceous film by thin-film deposition technology such as P—CVD and ion beam deposition instead of the above-described electroconductive paint film.
(D) Patent Document 5 discloses a method in which a substrate which is formed of stainless steel and which has carbonaceous particles adhered to its surface is heated. Since a diffusion layer is formed between the carbon particles and the substrate, the adhesion of the carbon particles is increased and the electroconductivity between the carbon particles and the substrate is improved.
(E) Patent Document 6 discloses a metallic separator having an electroconductive resin layer which is formed on the surface of a metallic substrate forming an electroconductive gas passageway. Carbon powder is dispersed in the electroconductive resin layer. In addition, Zr, Sn, Al, chromium-containing compounds, and/or molybdenum-containing compounds are disposed between the metallic separator and the electroconductive resin layer.    Patent Document 1: JP10-228914A    Patent Document 2: JP11-345618A    Patent Document 3: WO2003/44888    Patent Document 4: JP2000-67881A    Patent Document 5: WO99/19927    Patent Document 6: WO2001/18895    Patent Document 6: JP3365385B